1. Field of the Invention:
The invention relates to a treatment process limiting the overoxidation of steel, such as for example the sides of a coiled, hot-rolled sheet strip delivered from a strip mill.
2. Discussion of the Background
In iron-and-steel plants, cooling, prior to pickling, of strips of steel sheet prepared in the form of coils, requires substantial stocking areas. Cooling time in ambient air for a single coil is on the order of 70 hours. This time may be even longer in the event that the coils are superposed. In some plants, the coils are immersed in pools to accelerate their cooling. The advantages of this technique are to reduce the stocking area, the stocking time in particular between the strip mill and pickling, and the duration of pickling, the thickness of the scale forming on the steel strips being, less at the sides.
The scale formed at the time of hot rolling in the strip mill, between 800.degree. C. and 700.degree. C., consists of3 oxides which each have their own rate of growth.
Hematite, Fe.sub.2 O.sub.3, in contact with the atmosphere, is the most oxidized compound. For a scale of 10, .mu.m in thickness, formed at 700.degree. C., the thickness of the hematite film is on the order of 0.01 .mu.m.
FeO, in contact with the steel, is the least oxidized compound. For a scale of 10 .mu.m in thickness, the thickness of the FeO film formed is on the order of 7.26 .mu.m.
Between these two oxides, there is formed a compound of the spinel type, formed by the two preceding oxides. The thickness of this spinel, for a scale thickness of 10 .mu.m, is about 0.63 .mu.m.
The thickness of the scale formed on the hot-rolled steels depends on the dwell time on the cooling table, therefore on the coiling speed, on the rolling temperature and on the coiling temperature.
In the course of cooling in coil, at the sides, the oxygen present between the consecutive turns allows the continuation of oxidation, while this is checked rapidly at the center of the coil.
The center of the coil cools very slowly. The cooling speed is a function, in particular, of the mass of said coil. The decomposition of FeO sheltered from the air is practically complete; it is transformed into Fe.sub.3 O.sub.4 +Fe eutectoid.
The sides undergo an overoxidation according to the coiling temperature and the coil mass. This overoxidation occurs through the formation of a film of hematite the removal time for which determines the duration of pickling.
As the speed of cooling of the sides is more rapid, the decomposition of FeO into eutectoid is not complete, and the presence of magnetite is observed.
To promote adherence of the scale at the sides, it is necessary to avoid overoxidation and to approach the structure of the oxide forming the scale at the center of the coiled strip.
Cooling in a pool makes it possible to obtain this result but with the drawback on the one hand, of being burdensome and, on the other, of altering the mechanical characteristics of the steel which constitutes the strips.